A center cutting end mill has been previously developed as disclosed in U.S. Pat. No. 3,938,231 wherein square inserts are positioned with zero axial lead angle for axial plunge cutting or for radial feed. In one of the forms disclosed a pair of radially opposed inserts is employed, one of which is located with its inner arcuate corner extending across the center axis of the end mill to provide center cutting action while the other relatively outward insert finished the side wall surface in an axial plunge cut and provides the leading cutting edge for a radial feed cut.
While such end mill may be employed for limited depth axial plunging it is not entirely satisfactory for through drilling normal to the workpiece outer surface where the cutting edges would break out simultaneously over the entire end wall of the hole. Furthermore, the absence of any positive lead angle for axial cutting limits lateral stability for fast deep drilling operations and entire side edge contact of the outermost insert with the hole bore leads to chatter and undesirable wear in drilling depths exceeding the diameter of the cutter.
The prior art also includes indexable insert drills of the type shown in U.S. Pat. Nos. 3,540,323 and 3,963,365 which employ a pair of square inserts each having eight indexable cutting edges positioned so that the active lead cutting edge of one cuts the inner half while the other cuts the outer half of the hole. A negative lead angle on the inner insert and positive lead angle on the outer result in the lead corners of both inserts substantially coinciding at a radius of about one-half the drill radius. Lead angles in the order of 15.degree. to 45.degree. are employed. The straight cutting edge of the inner insert extends across center and in the latter patent, an improved version of the former, the lead corner of the inner insert extends axially ahead of the corresponding corner of the outer insert. The latter patent also teaches the necessity or desirability of providing one or more hard wear strips on the periphery of the drill body to cause the drill to advance in a substantially straight line and prevent the drill body from rubbing against and being worn by the peripheral surface of the hole. Such requirement would appear to be due at least in part to the use of a positive axial lead angle on the outer insert combined with a negative axial lead angle on the inner insert located in diametrically opposed relation so that reactive cutting forces normal to the cutting edge of both inserts would tend to deflect or subject the drill to forces having a component in the same direction toward one side of the drill.